The capacity of fiber optic communications channels is rapidly increasing, for example, to 1000 Giga-bits per second (Gbps) per channel, and the channel count is also increasing. This growth is expected to require increased optical amplifier bandwidth, increased levels of modulation format to transmit multiple bits per symbol and decreasing frequency spacing between channels. Increasing modulation format levels increases information capacity for a constant bandwidth but suffers a penalty of decreased signal to noise ratio, resulting in a shorter optical fiber length between regenerators. For example, a loss of about 50% in reach occurs for each additional bit of capacity obtained by increasing the number of bits per symbol. As another example, a change from Quadrature Phase Shift Keying (QPSK) to 16 level Quadrature Amplitude Modulation (QAM) results in approximately a 7 decibel (dB) penalty for a fixed symbol rate. Therefore, if a 100 Gbps QPSK signal propagates 1500 kilometers (km), then a 200 Gbps 16 level QAM signal will propagate only about 300 km. Decreasing frequency spacing between channels also results in a reach penalty due to increased non-linear cross-talk in a fiber.
Several methods for increasing capacity have been considered. Wavelength Division Multiplexing (WDM) using on-off keying with sub-carriers precisely locked to a data rate uses a receiver with optical filtering to select individual sub-carriers and uses direct detection for conversion to the electrical domain. Such a system has low tolerance for chromatic dispersion (CD) and polarization mode dispersion (PMD), and is unable to scale to phase-based or multi-symbol modulation formats.
Coherent Optical Orthogonal Frequency Division Multiplexing (OFDM) is essentially a direct application of wireless OFDM principles to the optical domain. “Virtual” sub-carriers with superimposed data modulation are generated in digital electronics by way of an inverse Fast Fourier Transform (FFT) in a transmitter. The data is recovered at a receiver by applying an FFT. This requires sophisticated digital signal processing and further requires adding redundant data to recover from channel distortion. Also, required bandwidth of opto-electronic components is high, and sub-carriers are of sufficiently low frequency to require complex phase recovery techniques due to their relative sensitivity to optical phase noise.
Sub-band multiplexed OFDM is an extension of OFDM concepts where several closely spaced OFDM channels form a quasi-continuous spectrum. This method requires complex digital signal processing, and power consumption of opto-electronic components is high. Further, sharp roll-off optical filters may be required in some implementations for sub-band separation.
Optically combined Binary Phase Shift Keyed (BPSK) signaling has been shown to be possible in conjunction with direct detection at a receiver. However, proper operation requires high opto-electronic device bandwidth encompassing a complete channel, which is impractical at speeds such as 1000 Gbps.
Single carrier modulated channels can be grouped close together or orthogonalized, in conjunction with a coherent receiver with Digital Signal Processing (DSP) for channel filtering, channel distortion compensation, and data decisions. However, the large amount of DSP required results in large chip sizes and high power consumption.
Another method for increasing capacity is optical Polarization Multiplexing (PM). By placing signals on each of two orthogonal polarizations of a light wave carrier the capacity can be doubled per wavelength. However, de-multiplexing the polarization-dependent signals at a receiver is a challenging task because the state of the polarization varies randomly along the length of the optical fiber that carries the light waves. Thus, conventional coherent detection results in an output that is a mixture of both polarization channels. Although real time optical polarization tracking can be achieved digitally in coherent receivers with high speed digital signal processing (DSP), DSP chips can consume significant power and pose design challenges when scaling to a channel capacities in the Terra-bits per second range. Real time optical polarization tracking at low cost, low complexity with low power consumption is desirable but has been unattainable thus far.